wellbore

A wellbore is a hole drilled or bored into the earth, typically to locate and develop subsurface resources or to obtain groundwater or perform geotechnical investigations. In petroleum engineering, the term usually refers to the fully assembled borehole—from the surface to the target formation—incorporating casing, tubing, and downhole equipment installed during completion.

During drilling, boreholes are stabilized with casing and cement to prevent collapse and to isolate fluids between formations. The outermost casing is cemented to create impermeable barriers. Drilling fluids, or muds, provide hydrostatic pressure, cool and lubricate the bit, and transport drill cuttings to the surface. The borehole is gradually enlarged through drilling, logging, and casing operations as the well is prepared for production or use.

Well construction typically involves multiple casing strings, cementing operations, and the installation of production tubing and downhole tools. Logging-while-drilling (MWD) and wireline logging measure formation properties, while perforating guns and pumps enable production. The completion design determines how fluids flow from the reservoir into the well and to surface facilities.

Wellbore integrity is essential and managed through proper design, mud programs, cementing quality, corrosion control, and surveillance. Common issues include borehole instability, casing deformation, cement debonding, and fluid leaks. Remedial actions can include re-cementing, mud remediation, or refracturing.

Applications include oil and gas wells, groundwater supply wells, geothermal wells, and carbon capture and storage wells. Abandonment and plugging are performed when a well is no longer usable, with cement plugs and mechanical barriers to permanently isolate the borehole from surrounding formations and the surface.